The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to a below-surface formation or subterranean zone of interest. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. The process also involves a drilling fluid system, which in most cases uses a drilling “mud” that is pumped through the inside of piping of the drill string to cool and lubricate the drill bit. The mud exits the drill string via the drill bit and returns to surface carrying rock cuttings produced by the drilling operation. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) the drill bit; 2) a steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment used in logging-while-drilling (“LWD”) and/or measurement-while-drilling (“MWD”) to evaluate downhole conditions as drilling progresses; 4) means for telemetering data to surface; and 5) other control equipment such as stabilizers or heavy weight grounding subs. The BHA is conveyed into the wellbore by a string of metallic tubulars (i.e. drill pipe).
MWD equipment is used to provide downhole sensor and status information to surface while drilling in a near real-time mode. This information is used by a rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, and hydrocarbon size and location. The rig crew can make intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real-time MWD data allows for a relatively more economical and more efficient drilling operation.
Known MWD tools contain essentially the same sensor package to survey the well bore; however the data may be sent back to surface by various telemetry methods. Such telemetry methods include, but are not limited to, the use of hardwired drill pipe, acoustic telemetry, use of fibre optic cable, Mud Pulse (MP) telemetry and Electromagnetic (EM) telemetry. The sensors are usually located in an electronics probe or instrumentation assembly contained in a cylindrical cover or housing, located near the drill bit.
MP telemetry involves creating pressure waves (“pulses”) in the drill mud circulating through the drill string. Mud is circulated from surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. The pressure pulses are achieved by changing the flow area and/or path of the mud as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the mud. The pressure differentials or pulses may be either negative pulses or positive pulses. Valves that open and close a bypass mud stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Pulse frequency is typically governed by pulse generator motor speed changes. The pulse generator motor requires electrical connectivity with the other elements of the MWD tool.
The pulse generating motor driveline system is subjected to extreme pressure differentials of about 20,000 psi between the external and internal aspects of the MWD tool when the MWD tool is downhole. To accommodate this large pressure differential, the mud is allowed access to areas of the MWD tool which are positioned on one side of a pressure compensation mechanism. Pressure is equalized on the other side of the pressure compensation mechanism within the tool using clean lubrication liquid, such as hydraulic fluid or silicon oil. One type of pressure compensation mechanism uses a flexible membrane positioned on a support surrounding a driveshaft of the MWD tool. The flexible membrane is typically attached to the support using wire and can flex in response to pressure differentials in the mud allowing pressure equalization between the mud external to the membrane and the lubrication liquid internal to the membrane.